Defining JavaScript variables inside if-statements
JavascriptVariablesIf StatementJavascript Problem Overview
Is defining JavaScript variables inside if-statements correct?
if(a==1){
var b = 1;
} else {
var b = 0;
}
I know the code above will work, however, WebMatrix highlights the variables.
Should I define the variables outside the if-statement? Or the first option's correct? Or it doesn't really matter?
var b = '';
if(a==1){
b = 1;
} else {
b = 0;
}
Javascript Solutions
Solution 1 - Javascript
As of the official release of ES2017 spec (2017-07-08), EcmaScript does support true block scope now using the let
or const
keywords.
Since ECMAscript doesn't have block scope but function scope, its a very good idea to declare any variable on the top of your function contexts.
Even though you can make variable and function declarations at any point within a function context, it's very confusing and brings some weird headaches if you aren't fully aware of the consequences.
Headache example:
var foo = 10;
function myfunc() {
if (foo > 0) {
var foo = 0;
alert('foo was greater than 0');
} else {
alert('wut?');
}
}
Guess what, we're getting a 'wut?' alert when calling myfunc
here. That is because an ECMAscript interpreter will hoist any var
statement and function declaration to the top of the context automatically. Basically, foo
gets initialized to undefined
before the first if statement
.
Further reading: JavaScript Scoping and Hoisting
Solution 2 - Javascript
Note that ECMAscript 6 does support block-level variables using the 'let' rather than the 'var' keyword. While variables declared with 'var' are hoisted to be function-scope regardless of where they are declared, those defined using 'let' are scoped to the enclosing block only.
Solution 3 - Javascript
Putting a var inside an if statement is not against "the rules" of the language, but it means that, because of var hoisting, that var will be defined regardless of whether the if statement's condition is satisfied.
Solution 4 - Javascript
See function four
on https://stackoverflow.com/questions/500431/javascript-variable-scope
As of 2012, there's no block-level scope in JavaScript. So your first version is fine: the variables are defined in the scope outside the if
block.
Solution 5 - Javascript
Because JavaScript's variables have function-level scope, your first example is effectively redeclaring your variable, which may explain why it is getting highlighted.
On old versions of Firefox, its strict JavaScript mode used to warn about this redeclaration, however one of its developers complained that it cramped his style so the warning was turned off. (Current versions of Firefox support a block-level variable declaration syntax.)